Spectral signature

1. SPECTRAL REFLECTANCE OF
VEGETATION, SOIL AND WATER
LWR314

2. Electromagnetic Spectrum
• Energy transfer from one body to another in the
form of electromagnetic waves
• A fundamental characteristic of radiation is the
wavelength ( ) of propagation
• All bodies radiate at a large number of wavelengths
(continuous range of wavelengths)

3. Spectral Reflectivity
• Reflectivity is the fraction of incident radiation
reflected by a surface
• The reflectance characteristics of Earth’s surface
features may be quantified by measuring the portion
of incident energy that is reflected
• This is measured as a function of wavelength (λ) and
is called spectral reflectance (rλ)

4. Typical spectral reflectance curves for
vegetation, soil and water

5. Spectral reflectance of Vegetation
• Chlorophyll strongly absorbs energy in the
wavelength bands centred at about 0.45 μm (blue)
and 0.67 μm (red)
• Our eyes perceive healthy vegetation as green in
colour because of the very high reflection of
green light
• If a plant is subject to some form of stress, it may
decrease chlorophyll production resulting in less
chlorophyll absorption in the blue and red bands
• Often the red reflectance increases to the point that
we see the plant turn yellow (combination of green
and red)

6. Colour Wheel

7. Spectral reflectance of Vegetation

8. Spectral reflectance of Vegetation
• In the range from about 0.7 to 1.3 μm a plant leaf
typically reflects 40 - 50% of the energy incident
upon it primarily due to the internal structure of
plant leaves
• Because the internal structure of leaves are highly
variable between plant species, reflectance
measurements in this range often permit us to
discriminate between species (even if they look the
same in visible wavelengths)
• Many plant stresses alter the reflectance in this
region, and sensors operating in this range are often
used for vegetation stress detection

9. Spectral reflectance of Vegetation

10. Spectral reflectance of Vegetation

11. Spectral reflectance of Vegetation
• Beyond 1.3 μm energy incident upon vegetation is
essentially absorbed or reflected with little to no
transmittance of energy
• Dips in reflectance occur at 1.4, 1.9 and 2.7 μm
because water in the leaf absorbs strongly at these
wavelengths (water absorption bands)
• Reflectance peaks occur at about 1.6 μm and 2.2 μm,
between the absorption bands
• Throughout the range beyond 1.3 μm, leaf
reflectance is approximately inversely related to the
total water present in a leaf which is a function of
both the moisture content and the thickness of a leaf

12. Changes in the reflectance from a
vegetative surface owing to water stress

13. Typical spectral reflectance curves for
vegetation, soil and water

14. Spectral reflectance of Soil
• The factors that influence soil reflectance act over
less specified spectral bands
• Factors affecting soil reflectance are moisture
content, soil texture (proportion of sand, silt and
clay), surface roughness, presence of iron oxide and
organic matter content
• The presence of moisture in soil will decrease its
reflectance - this effect is greatest in the water
absorption bands at about 1.4, 1.9, 2.2 and 2.7 μm
• Soil moisture content is strongly related to the soil
texture

15. Spectral reflectance of Soil
Variation in the spectral reflectance characteristics of soil according to
moisture content

16. Spectral reflectance of Soil
Variation in the spectral reflectance characteristics of soil according to
soil texture

17. Typical spectral reflectance curves for
vegetation, soil and water

18. Spectral reflectance of Water
• Water (in soil, vegetation or water bodies)
absorbs radiation at near-IR wavelengths and
beyond (strong absorption bands at about 1.4,
1.9 and 2.7 μm)
• Reflectance from a water body can stem from
an interaction with:
– the water’s surface (specular reflection),
– with material suspended in the water, or
– with the bottom of the water body

19. Spectral reflectance of Water
• Clear water absorbs relatively little energy with
wavelengths < 0.6 μm, resulting in high
transmittance in the blue-green portion of the
spectrum
• As the turbidity of water changes, the reflectance
changes dramatically
• Increases in chlorophyll concentration tend to
decrease reflectance in blue wavelengths and
increase it in green wavelengths (can monitor
algae)

20. Spectral reflectance of Water
Variation in the spectral reflectance characteristics of turbid water
according to the content of suspended solids